Crack-free hard chrome

ABSTRACT

CHROMIUM METAL, FORMED UNDER CERTAIN CONDITIONS OF CHEMICAL ACTIVATION DURING ELECTRODEPOSITION, HAS A HARDNESS IN EXCESS OF 700 VPH100 AND COMPLETE FREEDOM FROM CRACKS IN ITS STRUCTURE BOTH UPON INITIAL DEPOSIT AND UPON AGING FOR AT LEAST A YEAR.

July 31, 1973 I s. EISNER 3,749,652

CRACK-FREE HARD CHROME Filed Feb. 10, 1972 Fig. 2.

United States Patent CRACK-FREE HARD CHROME Steve Eisner, Schenectady, N.Y., assignor to Norton Company, Troy, N.Y. Filed Feb. 10, 1972, Ser. No. 225,218

The portion of the term of the patent subsequent to Nov. 9, 1988, has been diselaimed Int. Cl. C22d 1/00; C23b 5/06 U.S. Cl. 204105 R 1 Claim ABSTRACT OF THE DISCLOSURE Chromium metal, formed under certain conditions of mechanical activation during electrodeposition, has a hardness in excess of 700 VPH and complete freedom from cracks in its structure both upon initial deposit and upon aging for at least a year.

FIELD OF THE INVENTION Chromium metal has been used as a protective, wearresistant hard metal coating for functional as contrasted to decorative applications. Such coatings are deposited by electroplating from an aqueous solution.

DESCRIPTION OF THE PRIOR ART Chromium metal, hard enough to provide a wearresistant surface, has heretofore inherently had a crack pattern in the deposit. Commonly termed a mud crack pattern, the appearance of the deposit under magnification resembles sun-baked earth with a large number of cracks defining small islands of metal. While these cracks are usually obvious in the deposit immediately after formation, they will always be present in conventional deposits having a thickness of 3 microns or more and are fully developed after the deposit has aged for a relatively short time, e.g. not more than 56 days. These cracks are apparently due to tensile stresses which develop during the deposition.

Crack-free soft chrome, i.e. metal having a hardness of 600 VPH or less (Vickers Pyramid Hardness at a 100 gram load), can be produced usually by depositing with low current densities and high temperature baths using the conventional sulfuric acid: chromic acid ratio of 1:100.

However, according to the prior art the chrome deposit is either soft and crack-free or hard (700 VPH or more) and cracked. he hard, cracked chrome of the prior art is a good wear resistant surface but it is incapable of providing protection to a substrate where, for example, a highly oxidizing environment exists. This bars its use in many high temperature applications where steel or other substrates, e.g. molybdenum, are rapidly corroded by oxygen unless completely shielded by a protective coating.

SUMMARY The present invention is the provision of a chromium metal deposit which is not only crack-free but which has a hardness in excess of 700 VPH Further, this deposit maintains its crack-free nature upon aging for at least in excess of the 56-day period within which conventional hard chrome invariably forms cracks and, in fact, for many times this period (deposits aged for over 2 /2 years have retained their crack-free structure).

Deposits of this character are formed by the application of certain conditions of the process described and claimed in my prior U.S. Letters Pat. No. 3,619,384, issued Nov. 9, 1971. This permits formation of the crack-free deposit from an electrolyte which under conventional electrodeposition techniques will produce the conventional cracked hard chrome.

3,749,652 Patented July 31, 1973 DRAWINGS DESCRIPTION OF PREFERRED EMBODIMENTS The hard chrome deposits of the present invention will have a hardness of at least 700 VPH and generally will have a hardness in excess of 900 VPH The deposit, after polishing and etching, will show an essentially featureless surface, i.e. no visible crack lines. This crackfree structure will exist not only in thin deposits, e.g. 1 mil, but also in deposits of greater thickness, e.g. up to at least 6 mil in thickness. Tests have indicated that the structure will not develop cracks upon aging under normal atmospheric conditions for periods at least as long as two years and further that when deposits aged for this period of time are subjected to quite severe stress, for example, prying the deposit from the substrate to which it was applied, they remain resistant to crack development.

Reference to the illustrations of FIGS. 1 and 2 shows the visual difference between a conventional hard chrome surface (FIG. 1) and the surface of a deposit of the character of the present invention (FIG. 2). In FIG. 1, it will be seen that a multitude of intersecting crack lines criss-cross the surface. The measured crack density in this particular specimen was 1150 cracks lines/inch. FIG. 2, on the other hand, shows a complete absence of crack lines under the same magnification as that of FIG. 1 even though etching was carried out for more than 50% longer than for the sample of FIG. 1. Both samples were produced from conventional electrolyte baths of sulfuric acid-chromic acid which are known to produce only cracked deposits according to the prior art. Both samples were etched in the same way and under the same conditions except that etching was carried out in the case of FIG. 2 for 9 seconds longer than the 16 second etch of FIG. 1. The etching solution was a 33 oz./gal. chromic acid solution with a copper cathode at 6.75 volts and 1.20 amps used in both instances. The hardness of the specimens was measured by the standard Vickers Pyramidal Hardness test under a gram load (Metals Handbook, 8th ed.; vol. 2, p. 471). The hardness of the conventional cracked chrome of FIG. 1 was 1050 VPH while the hardness of the chrome sample according to the present invention (FIG. 2) was 964 VPH The sample of FIG. 1 was 0.0018 inch thick while the sample of FIG. 2 was 0.0052 inch thick. According to the prior art, thicker deposits are expected to develop crack patterns more rapidly than thin deposits. The deposit of FIG. 2 was approximately 2 /2 years old at the time the photomicrograph was taken and had been chipped mechanically from the steel substrate on which it was deposited about one week prior to the taking of the photomicrograph. The deposit of FIG. 1 was less than two years old and was separated from its substrate in the same way and at the same time as that of the FIG. 2 deposit.

The following example is illustrative of the procedure 384 and illustrated in Figure 3 of such patent, a 1 inch diameter steel rod end was contacted under a pressure of one pound per square inch by a non-woven porous mechanical activator disc formed with Dacron fibers. The disc carried a coating of 600 grit silicon carbide abrasive particles secured to the Dacron fibers by a polyurethane adhesive. The disc was rotated against the end of the rod during electrodeposition with an average speed of 1000 surface feet per minute measured at the center of the 1 inch rod face. The plating solution was a conventional chrome bath consisting of 0.44 oz./gal. sulfuric acid and 44.0 oz./gal. chromic acid. Plating was carried out for 15 minutes at a temperature of 130 F. to produce a deposit of chrome on the rod end 0.0052 inch thick. Hardness of this deposit was 964 VPH A photomicrograph of the surface of this specific deposit is shown in Figure 2 and was obtained as described above.

While the conditions of formation illustrated in the above example are considered to be the preferred conditions, variations in speed, pressure and hardness of the mechanical activator can be made if desired. The degree of mechanical activation should generally be not less than that exemplified above, and particularly the pressure of the activator against the deposit should not be dropped below about 1 p.s.i.

Deposits of the character of the hard chrome of the present invention will be useful wherever high temperatures are involved along with oxidation and subsequent spalling or disintegration of metals. Hard, crack-free chrome will provide a protective coating preventing such oxidation and its attendant problems. Molybdenum and alloys thereof used in the aerospace industry are typical problems of this type. Also, by applying the deposit of the present invention, inexpensive metal substrates, e.g. low carbon steel, can be upgraded to resist high temperature applications. Chemical attack also is protected against by this type of coating permitting such applications as extending the life of electrical contact rolls used in high speed strip platinglines. In addition to the oxidation and corrosion protection, deposits of this invention give wear, galling and erosion resistance to metal parts plated therewith.

The exact mechanism of the formation of the crackfree hard chrome deposit is not known. It is theorized that compressive stresses put in by a sufficient degree of mechanical activation off-set the normal tensile stresses and produce an apparently stress-free or compressively stressed hard chrome deposit.

I claim:

1. A crack-free electrodeposit of chromium metal having a hardness in excess of 700 VPH produced by activation of the deposit throughout its period of formation by relative motion and contact between such deposit and a plurality of small activating particles.

Referemes Cited I. E. Stareck et al.: 37th Annual Proceedings, American Electroplaters Soc., pp. 38-41, (1950).

Paul Morisset et al.: Chromium Plating, pp. 256, 364 and 365, (1954).

Samuel L. Hoyt: "Metals and Alloys Data Book, p. 275, (1943).

L. A. Smith: U.S. Bur. Mines, Information Circ. 6566, p. 2, April 1932.

GERALD L. KAPLAN, Primary Examiner US. Cl. X.R.

29194, 196.6; 204-51, DIG. 10 

